When my now-husband and I were engaged, his 4-year-old came to me, put his hands on his hips, and said "My mommy says YOU ARE NOT MY MOMMY!" I smiled and said "Your mommy is so right! She will always be your mommy. I am going to be your STEPmommy, and that means I get to love you and be here for you and help your daddy take care of you." Then I changed the subject.
A few days later, we went to dinner with some of my extended family. I overheard my stepdaughter telling my stepmother "<FDM> says that she is MY stepmommy and that means she gets to love me and be here for me and help my daddy take care of me." Her little face was just beaming. From that moment, that kid was mine.
For my bioson, I have two -
1) One day during my divorce that everything just overwhelmed me, and I was sitting under my desk crying. My 15-month-old crawled under the desk, climbed in my lap, wrapped his arms around me, and started rubbing my back the way I rubbed his when he cried. It was so exactly what I needed, and so much more special coming from a baby.
2) When he was 5, we went to DisneyLand. When we entered Cars Land, his eyes were so big. We had watched Cars so many times we could all recite it. We waited in line to "meet" Mater, and they told us to hang on just a sec. Lightning McQueen drove up, and my son ran over and threw himself on the hood, with his arms spread wide to give the car a hug. In his 9 years, I have never seen him that excited about anything. It was literally his dream come true.
When my oldest was 2, we went to the park at least once a week and played on the "little kid" playground. Some "big kids" were climbing the little rock wall at the other end of the "big kid" playground. My daughter just stood there and watched them with wide eyes for about 10 minutes. I turned to talk to another mom (since my kid hadn't moved in a while), and when I turned back, my baby was halfway up the rock wall. I went to grab her off and she yelled "NO" and clung tighter. She got all the way up, then let go and fell into my arms. I was so proud of her and so scared all at the same time. Even now, 10 years later, she's the same way - quietly observe until she's figured out what people are doing, then do it, all the while counting on me to be there in case something goes wrong.
Now that my kids are all older, I think what all of us will remember most are the evenings playing board games or cuddled on the couch watching a movie.